The present invention relates generally to coating formulations and methods of application of the same for rendering metallic parts corrosion resistant. Specifically, the present invention relates to a novel method and coating composition wherein a two-phase formulation containing a separate paint phase and a separate oil phase are simultaneously applied to the metal part in one step to provide improved resistance to corrosion.
Prior to the present invention, the conventional corrosion resistance treatment consisted of the steps of a conventional and well-known phosphating treatment of the metal part, followed by a painting step to apply a paint coating. After appropriate baking or drying of the paint coat, the metal part is immersed in an oil formulation to apply a coat of oil over the paint undercoat. Such treated metal parts must meet certain industry standards, such as set by the automotive industry for example, relating to corrosion resistance in salt spray, tests to be suitable for use.
The standard test generally employed in a salt spray corrosion test is ASTM Method B 117 wherein the treated metal part is exposed to a salt spray atmoshpere for a predetermined time. Metal parts treated in the prior art manner have shown reasonably acceptable degrees of corrosion resistance pursuant to industry standard salt spray tests, although in many instances two coats of paint followed by a coat of oil are required to assure the metal parts satisfactorily meet the minimum requirements.
U.S. Pat. No. 4,165,242 discloses a general description of prior art processes for treating phosphated metal parts to improve corrosion resistance utilizing the general steps of applying a paint coat and then an oil coat in separate steps and includes descriptions of some of the paint and oil formulations useful in such processes as is well-known to those skilled in the art.
However, prior to the present invention, there has been no formulation or method which consistently provides salt spray corrosion resistance equal to or exceeding automotive industry standards for painted parts without employing both a paint coating and a separate oil coating step.
Prior attempts to use an oil coupled into the paint vehicle gave limited corrosion resistance which did not acceptably meet industry standards. Therefore, prior to the present invention, the conventional paint undercoat and oil overcoat were applied in either a two or three step process.
The three step process referred to above involves a process wherein a large number of small parts are treated in a batch method. Two separate painting steps are employed wherein a first paint coat is applied to the parts. Then the paint step is repeated prior to application of the oil coat. This additional paint coat step is often required because of the tendency for the newly painted parts to adhere to one another during the baking step. After the parts are baked, any parts adhered to one another must be physically separated. However, often the paint coat is also pulled away from one of the parts at the point of engagement, resulting in exposing an unpainted surface area. To combat this problem, a second painting step is employed which tends to assure coverage of most of these exposed surfaces. Without this second painting step, the percentage of failed parts is often too high to acceptably meet automotive specifications for corrosion resistance.